As methods for measuring the intensity of an electromagnetic field due to an electromagnetic wave radiating from an electronic device to cope with EMI (electromagnetic interference), the following ones are set forth. For instance, the electronic device to be subjected to measurement, i.e. the sample device, is installed in an open space, and a loop antenna and a dipole antenna are installed at a distance of 3 m to 10 m from this sample device to carry out measurement. Where the antennas are installed at a sufficient distance from the sample device in this way, the loop antenna can measure only the magnetic field component of the distant radiating electromagnetic field while the dipole antenna can measure only the electric field component. Once one of the components of the distant radiating electromagnetic field is measured, the other can be calculated. There is also set forth a method by which measuring can be done in not only an open space but also an anechoic chamber.
On the other hand, in some cases, the radiation source of the electromagnetic wave is identified by the sample device. For instance, it may be determined on the circuit board what region the electromagnetic wave is powerfully radiated from. In such a case, unlike in the above-described case of measurement, the electromagnetic field intensity is measured in the vicinity of the sample device. Usually, a small loop antenna is brought close to the sample device to measure the magnetic field component of the electromagnetic field. Thus, the magnetic field component of the electromagnetic field attributable to the sample device is measured by utilizing a dielectric electromotive force due to inductive coupling. Whereas the magnitude and phase of signals are calculated by putting the magnetic field component thereby measured to arithmetic operation, instruments according to the prior art for measuring the magnetic field component in this manner include vector network analyzers and vector signal analyzers. One or another of such instruments is used to assess the characteristics of sensors and measure the distribution of harmonics in ICs, on the basis of which the current and voltage distributions in the sample device are figured out to identify the radiation source.
Incidentally, the aforementioned measuring method using an open space or the like requires a vast installation space and a large amount of facility investment. In view of this problem, an evaluation method using a coaxial transmission line known as a TEM cell has come to attract notice for the evaluation of the intensity of radiating electromagnetic waves. According to this evaluation method, the sample device is arranged between the internal conductor and the external conductor of the coaxial transmission line, and the evaluation is made according to the output from one end of the internal conductor. This method has the advantage of permitting evaluation with a relatively small facility.
However, the method using the TEM cell involves the problem of impossibility to correlate its measurements with those in an open space. Thus, because the distance between the sample device and the internal conductor is so short, the output current from the TEM cell cannot be considered-negligible without regard to the current due to inductive coupling and that due to capacitive coupling.
On the other hand, in order to identify the radiation source of the electromagnetic wave accurately by using the loop antenna, it is necessary to eliminate any influence of the electric field component. A shielded loop antenna, which is a loop antenna provided with a shield, is frequently used. Since this shielded loop antenna is hardly influenced by the electric field component, it is possible to measure only the magnetic field component with relatively high precision.
However, even with a shielded loop antenna it is difficult to measure only the magnetic field component accurately because an unshielded part of its structure is subject to electric field coupling with the sample device. Furthermore, the structural feature of having a shielded part makes it difficult to reduce the size of the antenna. Thus, it is difficult to improve the resolution. Also, since signals radiated from the sample device are actually unstable in frequency and some of them are modulated signals, it is extremely difficult to measure the phases of these signals and their electromagnetic field components.
An object of the present invention, attempted in view of the circumstances noted above, is to provide a measuring method for electromagnetic field intensity and an apparatus therefor, a measuring method for electromagnetic field intensity distribution and an apparatus therefor, and a measuring method for current and voltage distributions and an apparatus therefor, all capable of easily and accurately measuring with a compact and simple facility each of the electric field component and the magnetic field component of the electromagnetic field formed in space.
Another object of the invention is to provide a measuring method for electromagnetic field intensity and an apparatus therefor, a measuring method for electromagnetic field intensity distribution and an apparatus therefor and a measuring method for current and voltage distributions and an apparatus therefor, all permitting ready and reliable realization of the measurement of phases of signals radiated from the sample device, even if they are unstable in frequency as referred to above.